Role of fetal head‐circumference‐to‐maternal‐height ratio in predicting Cesarean section for labor dystocia: prospective multicenter study

ABSTRACT Objective To evaluate the relationship between the fetal head‐circumference‐to‐maternal‐height (HC/MH) ratio measured shortly before delivery and the occurrence of Cesarean section (CS) for labor dystocia. Methods This was a multicenter prospective cohort study involving four tertiary maternity hospitals. An unselected cohort of women with a singleton fetus in cephalic presentation, at a gestational age beyond 36 + 0 weeks and without any contraindication for vaginal delivery, was enrolled between September 2020 and November 2021. The MH and fetal HC were measured on admission of the patient to the labor ward. The primary outcome of the study was the performance of the HC/MH ratio in the prediction of CS for labor dystocia. Women who underwent CS for any indication other than failed labor progression, including fetal distress, were excluded from the final analysis. Results A total of 783 women were included in the study. Vaginal delivery occurred in 744 (95.0%) women and CS for labor dystocia in 39 (5.0%). CS for labor dystocia was associated with shorter MH (mean ± SD, 160.4 ± 6.6 vs 164.5 ± 6.3 cm; P < 0.001), larger fetal HC (339.6 ± 9.5 vs 330.7 ± 13.0 mm; P < 0.001) and a higher HC/MH ratio (2.12 ± 0.11 vs 2.01 ± 0.10; P < 0.001) compared with vaginal delivery. Multivariate logistic regression analysis showed that the HC/MH ratio was associated independently with CS for labor dystocia (adjusted odds ratio, 2.65 (95% CI, 1.85–3.79); P < 0.001). The HC/MH ratio had an area under the receiver‐operating‐characteristics curve of 0.77 and an optimal cut‐off value for discriminating between vaginal delivery and CS for labor dystocia of 2.09, which was associated with a sensitivity of 0.62 (95% CI, 0.45–0.77), specificity of 0.79 (95% CI, 0.76–0.82), positive predictive value of 0.13 (95% CI, 0.09–0.19) and negative predictive value of 0.98 (95% CI, 0.96–0.99). Conclusions In a large cohort of unselected pregnancies, the HC/MH ratio performed better than did fetal HC and MH alone in identifying those cases that will undergo CS for labor dystocia, albeit with moderate predictive value. The HC/MH ratio could assist in the evaluation of women at risk for CS for labor dystocia. © 2022 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.


INTRODUCTION
Labor dystocia is estimated to account for half of all primary Cesarean sections (CS) 1 and represents a risk factor for maternal and neonatal morbidity [2][3][4] . Labor dystocia may result from a relative or absolute mismatch between the size of the birth canal and that of the fetus 5 . The identification of women at risk for obstructed labor is among the main, and so far, unresolved, goals of antenatal evaluation.
Over the years, several studies have investigated the role of maternal and fetal anthropometric characteristics, such as maternal height (MH) [6][7][8][9] and fetal head circumference (HC) [10][11][12][13][14] and size [15][16][17][18] , in determining the risk of labor dystocia leading to CS. MH can be evaluated easily and has been highlighted as a possible surrogate for the size of the birth canal 19,20 and an indicator of the risk of CS secondary to labor dystocia [6][7][8][9] . With respect to fetal anthropometrics, as the fetal head must negotiate the maternal pelvic inlet, a large fetal HC has been proposed as a risk factor for labor dystocia leading to CS [10][11][12][13][14] . Nonetheless, to the best of our knowledge, no studies have evaluated a combination of maternal and fetal anthropometrics in relation to the risk of CS secondary to labor dystocia. We propose the ratio between the fetal HC and the MH (HC/MH ratio) as a more reliable predictor of obstructed labor compared with the constituent parameters on their own. The aim of this study was to evaluate the relationship between the HC/MH ratio calculated shortly before delivery and the occurrence of CS for labor dystocia.

METHODS
This was a multicenter prospective observational study conducted at four tertiary maternity hospitals, in Italy (University Hospitals of Parma, Rome Tor Vergata and Turin) and Germany (St Joseph Krankenhaus, Berlin), between September 2020 and November 2021. A non-consecutive series of singleton pregnancies with a cephalic-presenting fetus, gestational age (GA) ≥ 36 + 0 weeks and no contraindications for vaginal delivery were considered eligible for the study. Patients were approached on admission due to labor induction, premature rupture of the membranes or spontaneous labor. Informed consent for study participation was obtained upon enrolment. In all included cases, the fetal HC was measured on transabdominal ultrasound in the standard transventricular plane 21 , including the fetal scalp surrounding the fetal calvaria, within 72 h before delivery. Women scheduled for elective CS were excluded from the study.
A common protocol for labor management was shared across the participating units during the study period. Labor dystocia was defined based on the American College of Obstetricians and Gynecologists (ACOG)/Society for Maternal-Fetal Medicine (SMFM) recommendations for the safe prevention of primary CS 22 . In detail, a protracted active phase of labor was defined as ≥ 6 cm of dilatation with ruptured membranes and failure to progress despite 4 h of adequate uterine activity or at least 6 h of oxytocin administration with inadequate uterine activity. Arrest of dilatation in the first stage requiring CS was diagnosed following two more hours of oxytocin administration with no cervical change. A diagnosis of arrest of labor in the second stage was made in the event that the duration of the active phase was at least 2 h in parous women and 3 h in nulliparous women on epidural analgesia, or 1 h and 2 h in parous and nulliparous women without epidural analgesia, respectively.
According to the local protocol of the participating units, women diagnosed with labor dystocia underwent clinical examination by the senior obstetrician responsible for patient care. Obstetric intervention for labor dystocia was performed when the criteria for arrest of dilatation or arrest of labor in the second stage were fulfilled 22 . The obstetricians in charge of labor care were blinded to the measurement of fetal HC shortly before birth. Cases in which CS was performed for any reason other than labor dystocia, such as suspected intrapartum fetal distress, were excluded from data analysis.
Clinical data including maternal age, ethnicity, parity, height and body mass index (BMI), as well as GA at inclusion, mode of delivery and indications for CS, and neonatal outcomes, including birth weight, umbilical artery pH and 5-min Apgar score, were extracted from patient case notes. The HC/MH ratio was computed as the ratio between the measurement of the HC, expressed in mm, and that of the MH, expressed in cm. All information was collected after birth using a dedicated form. Data were recorded and stored in a Microsoft Excel secured pseudonymized database (Microsoft Corp., Redmond, WA, USA), which was accessible only to members of the research team. The primary outcome of the study was the relationship between the HC/MH ratio within 72 h before birth and the occurrence of CS for labor dystocia.
Statistical analysis was performed using Statistical Package for Social Sciences (SPSS) version 22 (IBM Corp., Armonk, NY, USA). The Kolmogorov-Smirnov test was used to assess the normality of the distribution of the data. The chi-square test was used to compare categorical variables and Student's t-test or the Mann-Whitney U-test was used as appropriate for continuous variables. Results are presented as n (%), mean ± SD or median (range). Logistic regression analysis was used to control for potential confounding variables. The performance of MH, HC and the HC/MH ratio in predicting CS for labor dystocia was determined using receiver-operating-characteristics (ROC)-curve analysis. The DeLong method was used for the comparison of areas under the ROC curve (AUCs). P < 0.05 was considered statistically significant. The study was approved by the local ethics committee of each participating unit and was reported in accordance with strengthening the reporting of observational studies in epidemiology (STROBE) guidelines 23 .

RESULTS
Overall, 836 women were eligible over the study period. Of these, 15 were excluded due to failed induction of labor and 38 due to CS for suspected intrapartum fetal distress, leaving 783 cases for data analysis (Figure 1). Among the included cases, vaginal delivery occurred in 744 (95.0%) women and CS for labor dystocia in 39 (5.0%).

Main findings
This study demonstrates that the HC/MH ratio is associated independently with CS for labor dystocia. The HC/MH ratio had the greatest accuracy in identifying cases that would undergo CS for labor dystocia, albeit with poor predictive value in our population of women at low risk for labor dystocia leading to CS.

Interpretation of findings and comparison with previous studies
Several studies suggest an association between MH and mode of delivery, including CS for labor dystocia [6][7][8][9] . This may be due to the fact that short women tend to have a proportionately narrow pelvis 19,20 , however, environmental factors which are recognized determinants of MH may also impact the occurrence of adverse labor outcome 24,25 . Another proposed explanation is that short women are more likely to be overweight or obese 26,27 . Our data confirm the association between short MH and CS for labor dystocia. Previous studies evaluating antenatal pelvimetry on ultrasound [28][29][30] or magnetic resonance imaging (MRI) 31,32 have demonstrated improved performance of direct pelvimetry compared with indirect assessment, as is the case with MH.
The influence of fetal size on labor outcome has also been studied extensively 15,16 . A high estimated fetal weight (EFW) represents a risk factor for CS for labor dystocia 17,18 , however prenatal ultrasound assessment of EFW has been shown to have limited accuracy in the prediction of fetal overgrowth [33][34][35] . Recent studies have suggested that the fetal HC represents a more reliable predictor of obstructed labor compared with EFW [10][11][12][13][14]36 . Our results demonstrate that the HC/MH ratio performs better than MH and fetal HC alone in predicting CS for labor dystocia, thus supporting the theory that labor dystocia is not an isolated maternal-or fetal-driven phenomenon, but may result from the interaction between maternal and fetal anthropometrics.

Clinical implications and future perspectives
The present study suggests that predictive parameters or models aiming to identify those laboring women at risk for labor dystocia leading to CS should ideally include maternal and fetal anthropometric characteristics.
Our results demonstrate a fair specificity and NPV of the HC/MH ratio, meaning that women with a HC/MH ratio below the index threshold have a good chance of delivering vaginally. Nonetheless, our data indicate that the HC/MH ratio has a poor sensitivity and PPV for CS for labor dystocia. On one hand, this can be explained by the fact that the size of the fetus and the pelvis do not represent the only determinants of obstructed labor, as malposition, cephalic malpresentation and asynclitism are among the factors which may cause a relative mismatch between size of the fetus and that of the birth canal [37][38][39] . On the other hand, our results need to be interpreted in the context of the study population, who were at low risk for labor dystocia. Therefore, the actual performance of the HC/MH ratio in predicting CS for labor dystocia should be investigated in future prospective studies in women diagnosed with labor dystocia. In such cases, the prolongation of labor has been shown to be detrimental in terms of maternal and neonatal outcome [2][3][4] . Should our hypothesis that the HC/MH ratio could assist in deciding the mode of delivery in the context of labor dystocia be confirmed, then the HC/MH ratio may represent a useful and readily available tool with the potential to improve labor management and outcome.
Moreover, the HC/MH ratio may also support obstetricians evaluating women at risk for fetal macrosomia close to term. A randomized controlled trial demonstrated that labor induction before 39 gestational weeks improved labor outcome in women at risk for fetal macrosomia based on an EFW above the 95 th percentile for the given GA 40 . However, the antenatal HC/MH ratio and Cesarean delivery 97 prediction of fetal macrosomia on two-dimensional 33,34 or three-dimensional [41][42][43] ultrasound has shown poor accuracy, while fetal weight estimation by MRI 44 is unlikely to be readily available in an emergency setting. In this context, the HC/MH ratio may be superior to the ultrasound estimation of fetal weight in identifying those women who may benefit from early labor induction. This hypothesis also warrants investigation in prospective studies.

Strengths and limitations
The strengths of this study include its evaluation of a novel parameter and the prospective enrolment of a large cohort of cases. The HC/MH ratio demonstrated fair specificity and NPV for identifying cases with a good chance of vaginal delivery, which may have potential utility in the context of labor dystocia and the antenatal counseling of patients with a large-for-gestational-age fetus. Another advantage of the parameter is that it can be quantified easily and quickly, and hence be readily available, even in emergency settings.
Conversely, the non-consecutive enrolment of patients could be seen as a limitation of the study findings. Furthermore, our results may not be generalizable to all centers, given that the study was conducted across four academic centers sharing a common protocol for labor management. The inclusion of the fetal scalp in the measurement of fetal HC could be also seen as a limitation, as it is not the standard recommendation 21 . However, we decided to include the fetal scalp in our measurement because we were not interested in brain growth, but rather in the entire size of the fetal head. Finally, fetal malposition, head malpresentation and asynclitism as determinants of obstructed labor in the presence of favorable maternal and fetal anthropometrics [45][46][47][48][49][50][51] were not evaluated in this study.

Conclusions
In a large cohort of women at low risk of labor dystocia enrolled within 72 h before delivery, the HC/MH ratio performed better than did fetal HC and MH alone in identifying those cases that will undergo CS for labor dystocia, albeit with poor predictive value. Further studies are needed to evaluate the performance of the HC/MH ratio in high-risk settings, such as in the event of labor dystocia or antenatal suspicion of fetal macrosomia. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Métodos. El tipo de estudio fue de cohortes prospectivo multicéntrico en el que participaron cuatro hospitales materno-infantiles de tipo terciario. Entre septiembre de 2020 y noviembre de 2021se alistó una cohorte no seleccionada de mujeres con fetoúnico en presentación cefálica, a una edad gestacional superior a las 36+0 semanas y sin ninguna contraindicación para el parto vaginal. La AM y el PC del feto se midieron en el momento del ingreso de la paciente a la sala de partos. El resultado primario del estudio fue la eficacia del cociente PC/AM en la predicción de la CS por distocia del parto. Se excluyeron del análisis final las mujeres a las que se practicó una cesárea por cualquier otra indicación que no fuera el fracaso del proceso del parto, incluido el sufrimiento fetal.

Conclusiones.
En una gran cohorte de embarazos no seleccionados, el cociente PC/AM funcionó mejor que el PC del feto y la AM por sí solos a la hora de identificar los casos que se someterán a una cesárea por distocia del parto, aunque con un valor predictivo moderado. El cociente PC/AM podría ayudar en la evaluación de las mujeres con riesgo de cesárea por distocia del parto.